fix: Inner batch transactions never have valid signatures

[ximinez]

High Level Overview of Change

Introduces a fix amendment that will skip over a problematic and unnecessary block in checkValidity that will set flags indicating that an inner transaction has a valid signature. Inner transactions, by definition, never have valid signatures.

I don't think this is exploitable as such, because there are additional checks before an inner transaction is applied to the ledger, but why take that chance?

It updates some of the callers of checkValidity to skip over the function if the tfInnerBatchTxn flag is set, either by treating it as always valid, or always invalid, depending on the context. The remaining will always fail for an inner tx.

Introduces amendment fixBatchInnerSigs.

Context of Change

This discussion on the Lending Protocol implementation PR: https://github.com/XRPLF/rippled/pull/5270#discussion_r2373083204

Type of Change

• [X] Bug fix (non-breaking change which fixes an issue)

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 79.1%. Comparing base (40198d9) to head (6369651).

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #6069   +/-   ##
=======================================
  Coverage     79.1%   79.1%           
=======================================
  Files          836     836           
  Lines        71245   71248    +3     
  Branches      8318    8317    -1     
=======================================
+ Hits         56354   56362    +8     
+ Misses       14891   14886    -5     

Files with missing lines	Coverage Δ
src/xrpld/app/misc/NetworkOPs.cpp	69.8% <100.0%> (ø)
src/xrpld/app/tx/detail/Transactor.cpp	92.2% <100.0%> (+0.2%)	:arrow_up:
src/xrpld/app/tx/detail/apply.cpp	94.5% <100.0%> (+2.0%)	:arrow_up:

... and 6 files with indirect coverage changes

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